Add a chapter on custom formatting to the stagin area

Using cities like in json :-)
I have no idea where this could go in the structure since traits are a bit too advanced a topic for the 2nd chapter...

Do you think that

```
let (lat, lat_c) = if self.lat >= 0.0 { (self.lat, 'N') } else { (-self.lat, 'S') }
```

may look clearer here?